Control arrangement for press grinder

ABSTRACT

THE SPECIFICATION DISCLOSES A PRESS GRINDER FOR MECHANICALLY REDUCING WOOD TO WOOD PULP IN WHICH TWO OR MORE PRESS BOXES ARE ARRANGED ABOUT THE GRINDING WHEEL AND HAVE PRESSURE MEMBERS FOR PRESSING WOOD WORK MEMBERS AGAINST THE WHEEL. EACH PRESSURE MEMBER HAS A DOUBLE ACTING FLUID MOTOR, AND A SEPARATE HIGH PRESSURE PUMP IS PROVIDED FOR SUPPLYING EACH FLUID MOTOR IN THE ADVANCING DIRECTION. A LOW PRESSURE FLUID SYSTEM IS PROVIDED FOR ACTUATING THE FLUID MOTORS IN A PRESS CHANGE OVER WHEN A NEW WORK MEMBER IS INSERTED IN A PRESS BOX. WHEN ADVANCE MOVEMENT OF A PRESSURE MOVEMENT IS INTERRUPTED THE DISCHARGE FROM THE HIGH PRESSURE PUMP THEREFOR IS DIVERTED TO ANOTHER FLUID MOTOR TO INCREASE THE ADVANCING SPEED THEREOF, WHEREBY THE LOAD OF THE GRINDING WHEEL REMAINS SUBSTANTIALLY CONSTANT AT ALL TIMES.   D R A W I N G

United States Patent 1191 Thumm et a1.

1111 3,791,596 1451 F1 I12ji974 [54] CONTROL ARRANGEMENT FOR PRESS 2,965,315 12/1960 Lyall 241 34 GRINDER 3,314,615 4/1967 Hill .j. 241/34 [76] hwemors? Helm Thumm Primary Examiner-Granville Y. Custer, Jr.

Robert-Koch-Strasse 64, Gerd Atmmey Melvin A Crosby Ruessmann, Stemstrasse 4, both of 7920 Heidenheim/Brenz; Josef [57] ABSTRACT Kotitschke, Kappelstrasse 33, 7924 v Steinheim an of Germany The specification dlscloses a'press grmder for mechan- H.h ically reducing wood to wood pulp in which two or Flledi y 6,.1'971 more press boxes are arranged about the grinding [21] AppL No; 147,061 wheel and have pressure members for pressing wood work members against the wheel. Each pressure member has a double acting fluid motor, and a separate g" Appllcatmn o g a high pressure pump is provided for supplying each June 10-, 1970 Germany 7021704 fluid motor in the advancing direction- A low p ssu fluid system is provided for actuating the fluid motors [52] US. Cl. 241/34 during change over when a new work member is in- [51] Int. Cl. B02c 23/02 serted in a press box. When advance movement of a [58] Field of Search 241/33, 34, 35, 36, pressure movement is interrupted the discharge from 241/280, 282 the high pressure pump therefor is diverted to another fluid motor to increase the advancing speed thereof, [56] References Cited whereby the load of the grinding wheel remains sub- UNITED STATES PATENTS stantially constant at all times.

2,545,260 3/1951 Cole 241/34 X 15 Claims, 3 Drawing Figures i 2712"" 11111. 1X1 M1121 PATENTEDFEB12|974 3391; 59

INVENTORS HELM UT THUMM GERD RUESSMAN N JOSEF MOT ITSCHKE WWW'M I Pmmmr 121974 sum 2' or 2 Fig. 2

GER WHSMANM JQsEF KOT IT scH ME wm- W CONTROL ARRANGEMENT FOR PRESS GRINDER The present invention relates to a press grinder for the grinding of wood to form wood pulp. More particularly, the present invention relates to a control arrangement for a press grinder of the nature referred to having two or more press boxes with hydraulically operated pressure members in which the total load on the machine and, particularly, on the grinding wheel drive motor, is maintained substantially constant even during change over when one of the pressure members is retracted to permit a new wood workpiece to be ground to be inserted in the respective press box.

Press grinders of the nature referred to are rather large pieces of machinery and have a rather large grinding wheel driven by an electric motor and also have an individual pump for the fluid motor connected to the pressure member in each press box and with one or more electric drive motors for driving the pumps.

With a two press grinder in which two fluid motors are provided for actuating the pressing members in the respective press boxes, a single electric motor can be provided for driving both of the pumps. In the operation of, for example, a two press grinder, the wood work members in the press boxes are pressed against the periphery of the grinding wheel while the wheel is driven in rotation. When a pressure member in a press box is close to the circumference of the grinding wheel, due to the grinding down of the wood work member therein, the fluid motor connected thereto is reversed to retract the pressure member away from the grinding wheel. A new wood work member is then put into the respective press box and the pressure member is thereafter advanced to bring the new wood work member into operative engagement with the circumference of the grinding wheel.

When the pressure member is retracted away from the grinding wheel and up until the time it again presses a wood work member to be ground against the circumference of the grinding wheel, the fluid motor connected thereto is under a rather light load and not much power is consumed in retracting the pressure member from the grinding wheel and in again advancing it toward the grinding wheel to bring the new wood work member to be ground against the grinding wheel. The fluctuation in power consumption'of the press grinder that comes about during the aforementioned retracting and initial advancing of a pressure member is undesirable and it is much to be preferred to maintain the total load on the press grinder substantially constant, thereby to reduce to a minimum the fluctuations in the amount of power supplied.

There is known from the prior art, two press grinders having a grinding wheel driven by an electric motor and having individual high pressure pumps for the fluid motors connected to the pressure members in the respective press boxes and driven by a single pump motor which is variable in speed. For the retraction of the pressure members away from the grinding wheel and for the idle advance stroke thereof to bring a new wood workpiece into engagement with'the grinding wheel, there is provided a separate low pressure pump. A regulating device is provided which controls the speed of the motor for the high pressure pumps and. the regulating device, therefore, also controls the rate of advance of the pressure members toward the grinding wheel. The regulating device is so constructed and arranged that the electric power supplied to the electric motor driving the grinding wheel remains substantially constant at all times. In this manner, it is avoided that the operation of inserting a new work member in one of the press boxes will be accompanied by a large drop in the power supplied to the motor driving the grinding wheel. Such a drop in power would be accompanied by an increase in the power supply to the grinding wheel motor after the new wood work member was in operative engagement with the grinding wheel.

In the regulating device referred to, when a pressure member starts its return'stroke away from the grinding wheel, the speed of the motor for the high pressure pumps will increase and this will increase the delivery from the high pressure pump for the fluid motor connected to the pressure member which is still pressing a wood work member against the grinding wheel. The increase in pressure on the fluid motor will increase the pressure of the wood work member against the grinding wheel and the power required to rotate the grinding wheel will remain substantially constant even though the high pressure pump associated with the fluid motor of the idle pressing member will, itself, be idling. The same cycle of events takes place when the next press change over occurs.

The reversing of the fluid motor to retract the pressure member away from the grinding wheel when a press change over is initiated occurs rather rapidly and the change in speed of the high pressure pump motor to increase the pressure of the work member still being ground against the grinding wheel is always accompanied by a certain delay and, thus, the described regulating device does not entirely eliminate large power fluctuations to the grinding wheel drive motor. The duration of such fluctuations may be reduced by the regulating device referred to, but the fluctuations are, nevertheless, quite large and are most undesirable.

With the foregoing in mind, a primary objective of the present invention is the provision of a control arrangement for a press grinder of the nature referred to which substantially eliminates large fluctuations in the amount of power supplied to the grinder.

Another object of the present invention is the provision of a control arrangement for a press grinder of the nature referred to in which there is no sudden drop in the amount of power supplied to the grinder when a pressure member is retracted from the wheel and no great surge in the power supplied to the grinder when a new wood work member to be ground is brought against the periphery of the grinding wheel of the grinder.

Still another object of the present invention is the provision of a control arrangement according to the aforesaid objects and which is relatively simple and which does not require expensive and complex electrical controls.

The exact natureof the present invention and the several objects and advantages thereof will become more apparent upon reference to the following detailed specification taken in connection with the accompanying drawings, in which:

FIG. 1 schematically shows a control arrangement according to the present invention adapted to a two press grinder;

FIG. 2 shows a modification of the present invention wherein a single valve structure controls the supply of high pressure fluid to the fluid motors for the pressure members of the press boxes of the press grinder; and

FIG. 3 shows a still further modification in which three valves are employed in the high pressure fluid system.

BRIEF SUMMARY OF THE INVENTION In the present invention, the problem of maintaining a substantially constant load on the drive motor for the grinding wheel in a press grinder is solved by diverting the high pressure actuating fluid from one of the fluid motors for the press boxes of the press to another of the fluid motors when the pressure members associated with the one fluid motor is to be retracted so that a new wood member can be placed in the pertaining press box. Each press box of the grinder has a pressure member and each pressure member has a fluid motor connected thereto and each fluid motor is provided with an individual high pressure pump which supplies the respective motor while the respective pressure member is pressing a wood work member against the grinding wheel.

GENERAL DISCUSSION OF THE INVENTION According to the present invention if, for example, a two press grinder is involved having two press boxes, each with a pressure member having a respective double acting fluid motor connected thereto and with an individual high pressure pump for each fluid motor and with a common motordriving the two pumps then, when either pressure member is reversed, the delivery from the respective high pressure pump is diverted to the fluid motor pertaining to the other pressure member. Thus, when one pressure member is reversed, the supply of pressure fluid to the fluid motor pertaining to the other pressure member doubles so that there is substantially no change of speed of the high pressure pumps and the increase in the fluid supply to the other fluid motor, and which occurs with extreme rapidity, maintains the grinding wheel at substantially the same total load as before so that there is substantially no fluctuation in the power supply to the electric motor driving the grinding wheel.

After the press change is completed and a new work member is placed ahead of the retracted pressure member, the pressure member is, advanced until the new work member is against the grinding wheel and the high pressure pump pertaining to the respective fluid motor is then returned thereto and both pressure members will advance at normal speed. This last mentioned change over also occurs rapidly and leads to only insignificant fluctuations in the load on the grinding wheel motor and, therefore, insignificant fluctuations in the power supply to the drive motor for the grinding wheel.

According to the present invention, each pump can be connected to its fluid motor by way ofa conduit system having a valve therein with the valve having one shifted position where the pump delivery is supplied to the respective fluid motor and another shifted position wherein the pump delivery is directed to another of the fluid motors. Such valves can be conventional in structure and are, therefore, quite economical.

Another possibility according to the present invention is to locate a shut-off valve between each pump and the respective fluid motor and to provide a transverse conduit leading between the discharge sides of the fluid pumps and also having a shut-off valve therein. Thus, when either shut-off valve in series with a fluid motor is shut off, the shut-off valve in the transverse conduit is opened and the fluid from the pump pertaining to the closed shut-off valve will be diverted to the other fluid motor. Such shut-off valves are also substantially conventional and are quite economical.

It is preferable to provide a common drive for the pumps referred to so that the speed of the pumps will be identical and the fluid motors driven thereby will also move at the same speed and in this manner a uniform quality of the wood particles will be produced by the grinder.

It is also possible, according to the present invention, to provide a regulating device for controlling the speed of a common drive motor for the pumps, or of several drive motors for several pumps, and to control the regulating device in conformity with the power supplied to the drive motor for the grinding wheel. Such a device, for example, might be sensitive to the current flowing to the electric motor for the grinding wheel. The application of the present invention makes a regulation device of the nature referred to unnecessary under normal operating conditions. However, there can always be substantial variation in the quality of the woods being ground, and it is contemplated that a press grinder embodying the control arrangement of the present invention could also embody a regulating device of this nature.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings somewhat more in detail, in FIG. 1, the cylindrical grinding wheel, or grindstone, 10 rotates on a horizontal axis and is driven by a nonillustrated electric motor. Arranged on opposite sides of grinding wheel 10 are two boxes 11 and 21 having therein the pressure members 12 and 22, respectively, and connected to each of which is a double acting hydraulic motor 13 and 23, respectively.

Each fluid motor has an advancing side facing away from the grinding wheel and responsive to a supply of fluid pressure thereto for moving the respective pressure member toward grinding wheel 10 to press a wood work member against the grinding wheel. Each fluid motor also has a retracting side facing the grinding wheel and responsive to a supplyof fluid pressure for retracting the pressure member away from the grinding wheel.

Each of the fluid motors 13,23 has a respective high pressure supply pump 14, 24, advantageously driven by a common motor 9. Pump 14 supplies pressure fluid to a conduit 15 and to the inlet port of a valve 16 which has a first outlet port connected by the conduit 17 through a check valve 17b with the advancing side of fluid motor 13. Valve 16 has a second outlet port to which is connected a branch conduit 27a which is connected to conduit 27 and which leads to the advancing side of fluid motor 23.

Similarly, pump 24 discharges into conduit 25 leading to the inlet port of a valve 26 having a first outlet port which is connected by conduit 27 through a check valve 27b to the aforementioned advancing side of fluid motor 23. According to conventional practices a pressure limiting valve is connected between conduits 15 and 25 and the reservoir from which pumps 14 and 24 draw fluid.

Valve 16 has a first shifted position in which conduit 15 is connected with conduit 17, while the second outlet port of the valve is interrupted. The valve has a second shifted position in which conduit is connected to conduit 270, while the first outlet port, which is connected to conduit 17, is shut off. Similarly, valve 26 has a first shifted position in which conduit 25 is connected to conduit 27, while the outlet port connected to conduit 17a is shut off and a second shifted position in which conduit 25 is connected to conduit 17a, while the outlet port connected to conduit 27 is shut off.

There is further provided a low pressure pump 7 driven by a motor, such as a constant speed electric motor, and which discharges to a conduit 8. Conduit 8 is connected to a conduit 18 leading to an inlet port of a valve 19 and is also connected to a conduit 28 leading to the inlet port of a valve 29. Valve 19 has one outlet port connected to conduit 17 and another outlet port connected to the retracting side of fluid motor 13. Similarly, valve 29 has a first outlet port connected to conduit 27 and a second outlet port connected to the retracting side of fluid motor 23. Valve 19 has a valve member therein which is normally in a centered position and in which position the retracting side of fluid motor 13 is connected to the reservoir while the inlet port of the valve that is connected to conduit 18 and the outlet port that is connected to conduit 17 are shut off. The valve member and valve 19 is shiftable in one direction to connect conduit 18 to the outlet port leading to conduit 17, while connecting the retracting side of fluid motor 13 to exhaust. The valve member of valve 19 has anothershifted position in which conduit 18 is connected to the retracting side of fluid motor 13, while the outlet port which is connected to conduit 17 is connected to the reservoir.

Valve 29, similarly, has a valve member having a normally centered position in which the inlet port that is connected to conduit 28 and the port connected to conduit 27 are shut off, while the retracting side of fluid motor 23 is connected to the reservoir. This valve member has a first shifted position in which conduit 28 is connected to the outlet port leading to conduit 27, while the retracting side of fluid motor 23 is connected to the reservoir and another shifted position in which the inlet port connected to conduit 28 is connected to the retracting side of fluid motor 23, while the advancing side offluid motor 23 is connected to the reservoir.

Pump 7, similarly to pumps 14 and 24, may be pro vided with a pressure limiting valve to bypass the pump to the reservoir when fluid is not being drawn therefrom.

FIG. 2 shows merely the high pressure fluid portion of FIG. 1 together with the fluid motors 13 and 23. In FIG. 2, the pumps 14 and 24, as before, are driven by a common motor 9 and discharge into conduits 15 and 25, respectively, to respective inlet ports of a valve 30. Valve 30 has a first outlet port connected by conduit 17 with the advancing side of fluid motor 13 and a second outlet port connected by conduit 27 with the advancing side of motor 23.

Valve 30 has a valve member therein which is normally centered and in which centered position conduit 15 is connected to conduit 17 and conduit 25 is connected to conduit 27. When shifted in one direction, for example, to the left, the valve member of valve 30 connects both of conduits 15 and 25 to conduit 27, while shutting off the connection to conduit 17, whereas when shifted in the opposite direction, namely, to the right, both of conduits l5 and 25 are connected to conduit 17, while the connection to conduit 27 is shut off. The high pressure fluid circuit arrangement of FIG. 2 accomplishes the same result as the circuit of FIG. 1, except that one single valve instrumentality is employed.

In FIG. 3, an arrangement is shown wherein a valve 32 is interposed between conduits l5 and 17 and has a valve member which in one position interconnects conduits 15 and 17, while in another position interrupts the connection between the said conduits. Similarly, a valve 34 is interposed between conduits 25 and 27 and has a valve member therein which, in one position, connects conduit 25 to conduit 27 and in the other position interrupts the said connection between the conduits. In FIG. 3, there is a lateral conduit 37 extending between conduits 15 and 25 and having therein a valve 36 with a valve member which in one position is closed and in another position interconnects the two portions of conduit 37 with each other.

In the FIG. 3 arrangement, valve 36 normally occupies the position shown when valves 32 and 34 are in the illustrated position. At this time, both fluid motors are advancing under high pressure. Whenever either of valves 32, 34 is shifted to interrupt the advancing movement of the respective fluid motor, valve 36 is also shifted so that the high pressure fluid from the pump which is not now supplying its respective fluid motor is diverted through the lateral conduit to the conduit leading to the advancing side of the other fluid motor.

Both of the circuits of FIGS. 2 and 3 have a low pressure fluid system complement the same as that illustrated and described in connection with FIG. 1.

In operation, and with reference to FIG. 1, and assuming the machine to be in idle condition, valves 19 and 29 are shifted to the left and low pressure fluid from pump 7 is supplied to the retracting sides of both of hydraulic motors 13 and 23 and the pressure members 12 and 22 are in retracted position. At this time, any fluid from high pressure pumps 14 and 24 supplied to either one or the other, or both, of conduits l7 and 27 will be bypassed to exhaust.

Work members to be ground can now be loaded into boxes 11 and 21 between pressure members 12 and 22 and grinding wheel 10. Initial rapid advance'of the pressure members toward the grinding wheel can be accomplished by shifting valves 19 and 29 to their extreme righthand positions and in which position the fluid from low pressure pump 7, as well as the fluid from high pressure pumps 14 and 24, will be supplied to conduits 17 and 27. k

When the work members in the press boxes engage grinding wheel 10, valves 19 and 29 are shifted to the centered positions thereof which interrupts the connections between low pressure pump 7 and hydraulic motors 13 and 23. At this time, valves 16 and 26 are in the righthand shifted positions thereof and high pressure pump 14 supplies pressure fluid to conduit 17 while high pressure pump 24 supplies pressure fluid to conduit 27. Grinding of the work members in the press boxes now proceeds until one of the pressure members must be retracted, for example, pressure member 12.

When this becomes necessary,'valve 16 is shifted to its lefthand position thereby interrupting the connection between high pressure pump 14 and the advancing side of hydraulic motor 13 while valve 19 is shifted to its lefthand position thereby-exhausting the advancing side of hydraulic motor 13 while supplying fluid from low pressure pump 7 to the retracting side thereof and causing the hydraulic motor and pressure member 12 to retract. The aforementioned shifting of valve 16 diverts the fluid from high pressure pump 14 to conduit 27a and thence to conduit 27 where this fluid combines with the fluid from high pressure pump 24 and increases the supply of pressure fluid to the advancing side of hydraulic motor 23 thereby increasing the pressure with which pressure member 22 urges work members in box 21 toward grinding wheel 10 and maintaining the load on grinding wheel 10, and on the high pressure drive motor, substantially constant.

When press box 11 is again supplied with work members, and it is desired to advance pressure members 12 toward the grinding wheel, valve 19 is shifted to its righthand position thus exhausting the retracting side of hydraulic motor 13 while supplying fluid from low pressure pump 7 to the advancing side of motor 13. Pressure member 12 will thus advance toward grinding wheel 10 and bring the work members into engagement therewith. When the work members in box 11 engage grinding wheel 10, valve 19 is shifted back to its centered position thereby interrupting the supply of fluid from low pressure pump 7 to the advancing side of hyposition thereby interrupting the connection of high pressure pump 14 with conduit 27a and, instead, again connecting high pressure pump 14 with the advancing side of hydraulic motor 13 via conduit 17. The load on the advancing side of hydraulic motor 23 at this time drops because of the diminished supply of high pressure fluid thereto while the load on the advancing side of hydraulic motor 13 is picked up by the supply of high pressure fluid thereto.

It will be seen that, once the machine is set into operation, the individual boxes can be selectively refilled, whenever necessary, while the load on the grinding wheel, and on the drive motor for pumps 14 and 24, remains substantially constant.

Utilizing the same low pressure system involving the 7 low pressure pump 7 to effect retraction and rapid advanceof the fluid motors, the system leading from high pressure pump 14 and 24 to the advancing sides of hydraulic motors 13 and 23 can be modified as illustrated in FIGS. 2 and 3. In every case, the fluid from pumps 14 and 24 can be directed to respective ones of the hydraulic motors during working or to one only thereof during the recharging of the press box pertaining to the other of the said hydraulic motors.

It will be understood that, in a conventional manner, the low pressure pump develops a predetermined output pressure at a predetermined speed and decreases in rate of delivery as the pressure load thereon increases. The high pressure pumps, on the other hand, are substantially the same rate of delivery as the pressure load thereon increases.

For this reason, when both high pressure pumps are connected to a single one of the advancing sides of motors 13, 23, the supply of fluid to the respective motor is, as mentioned, doubled, thereby causing the respective motor to advance more rapidly and to maintain about the same working load on the grinding wheel and about the same load on the high pressure pump drive motor as when each of the advancing sides of the re- Modifications may be made within the scope of the appended claims.

What is claimed is:

1. In a press grinder-having a motor driven grinding wheel and operable for grinding wood members for the creation of wood pulp and having at least two press boxes at circumferentially spaced locations around the wheel adapted to receive wood members to be ground and with each box having a pressure member with a double action fluid motor connected thereto, each fluid motor having an advancing s'ide responsive to a supply of pressure fluid to urge the pertaining pressure member toward the grinding wheel and a retracting side responsive to a supply of pressure fluid to urge the pertaining pressure member away from the grinding wheel, a high pressure pump for each fluid motor, motor means to drive said pumps, conduit means connecting the discharge side of each pump with the advancing side of the respective fluid motor, and shiftable valve means in said conduit means operable in one shifted position to supply fluid from each pump to the advancing side of the pertaining fluid motor to urge the pertaining pressure member toward said wheel, said valve means in another shifted position interrupting the supply of fluid from one of said pumps to the advancing side of the respective fluid motor and instead supplying the said fluid from said one pump to the advancing side of another of the fluid motors to increase the supply of pressure fluid to the advancing side of said other fluid motor and thereby to maintain the working load on said grinding wheel and on said motor means driving said pumps substantially constant.

2. A press grinder according to claim 1 in which said valve means comprises an individual valve in series with each said pump on the discharge side thereof.

3. A press grinder according to claim 2 in which each said valve has an inlet port connected to the discharge side of the respective pump, a first outlet port connected to the advancing side of the respective fluid motor, a second outlet port connected to one of the discharge side of another said pump and the advancing side of the fluid motor pertaining to said other pump, and a valve member in the valve having a first shifted position in which said inlet port is connected to said first outlet port and a second shifted position wherein said inlet port is connected to said second outlet port.

4. A press grinder according to claim 1 in which said valve means includes a single valve having first and second inlet ports connected to the discharge sidesof first and second ones of said pumps and having first and second outlet ports connected to the advancing sides of the fluid motors pertaining to said first and second pumps respectively, said valve also having a shiftable valve member having a center position in which said first and second inlet ports areconnected to said first and second outlet ports respectively and having a first shifted position in which both of said inlet ports are connected to said first outlet port while said second outlet port is closed off, said valve member having a second shifted position in which both of said inlet ports are connected to said second outlet port while said first outlet port is closed off.

5. A press grinder according to claim 4 in which said valve member is normally in said center position, and

actuating means for shifting said valve member selectively into the said shifted positions thereof.

6. A press grinder according to claim 1 in which said valve means includes first, second and third valves each having a first shifted position wherein fluid will flow therethrough and second shifted positions wherein fluid flow therethrough is interrupted, each of said first and second valves being serially arranged in said conduit means between the discharge sides of first and second ones of said pumps and the advancing sides of the pertaining fluid motors, said third valve being connected between the discharge sides of the said first and second pumps, said third valve being adapted to occupy the said second position thereof when said first and second valves occupy the said first positions thereof and being shiftable into the said first position thereof substantially simultaneously with the shifting of either of said first and second valves into the said second position thereof.

7. A press grinder according to claim 1 in which said motor means comprises a single motor driving said pumps.

8. A press grinder according to claim 1 in which said grinder comprises two press boxes each having a pressure member and a fluid motor for each pressure member, a pump for each fluid motor, and said motor means comprising a single drive motor driving said pumps.

9. In a two press grinder: a motor driven grinding wheel, a pair of press boxes on opposite sides of said wheel to receive wood members to be ground, a pressure member in each box to press the wood members therein against the wheel, a double acting fluid motor connected to each pressure member having an advancing side facing away from said wheel and a retracting side facing the wheel, a motor driven high pressure pump for each fluid motor, and valve means connecting the discharge sides of said pumps to said advancing the sides of said fluid motors and operable selectively to supply the output from each of said pumps to a respective one of said advancing areas or to supply the outputs from both of said pumps to a selected one of said advancing areas.

10. A two press grinder according to claim 9 which includes further low pressure pump means, and valve means connected between the discharge side of said further pump means and the advancing and retracting sides of said fluid motors, said valve means including valve member means therein shiftable for reversibly supplying fluid to said advancing and retracting sides of said fluid motors and for connecting said retracting sides to exhaust while interrupting the connection between said further pump means and said advancing sides.

11. A two press grinder according to claim 10 in which said valve means comprises a valve connected between said further pump means and each said fluid motor, each valve having a valve member having a first position wherein the advancing side of the respective fluid motor is connected to exhaust and the retracting side of the respective motor is connected to said further pump means, said valve member having a second position wherein said connections are reversed and a third position wherein the retracting side of the respective fluid motor is connected to exhaust while the connection leading from the valve to the advancing side of the respective fluid motor is closed off.

12. A press grinder according to claim 1 which includes further low pressure pump means, and valve means connected between said further pump means and the advancing and retracting sides of said fluid motors and selectively operable for reversibly supplying fluid to each thereof from said further pump means while the supply of fluid from the first mentioned said pump pertaining to advancing side of the respective fluid motor is interrupted.

13. A press grinder according to claim 3 which includes further low pressure pump means, and valve means connected between said further pump means and the advancing and retracting sides of said fluid motors and selectively operable for reversibly supplying fluid to each-thereof from said further pump means while the supply of fluid from the first mentioned said pump pertaining to advancing side of the respective fluid motor is interrupted.

14. A press grinder according to claim 4 which includes further low pressure pump means, and valve means connected between said further pump means and the advancing and retracting sides of said fluid motors and selectively operable for reversibly supplying fluid to each thereof from said further pump means while the supply of fluid from the first mentioned said pump pertaining to advancing side of the respective fluid motor is interrupted.

15. A press grinder according to claim 6 which includes further low pressure pump means, and valve means connected between said further pump means and the advancing and retracting sides of said fluid motors and selectively operable for reversibly supplying fluid to each thereof from said further pump means while the supply of fluid from the first mentioned said pump pertaining to advancing side of the respective 

